Islamic State (IS) militants on November 13 released an audio recording of what the extremist group claims is the IS leader Abu Bakr al-Baghdadi, days after an air strike on jihadist leaders in Iraq generated rumors he had been wounded or killed.

In the 17-minute recording, a man purported to be Baghdadi made no direct reference to the November 8 strike but mentioned developments that have occurred since then.

There was no immediate confirmation about whether the recording was actually Baghdadi.

The voice on the recording said IS fighters "will never leave fighting, even if only one soldier remains."

He also said the U.S.-led military campaign against IS is failing, and that the "caliphate" declared by the IS has expanded to Saudi Arabia, Yemen, Egypt, Libya, and Algeria.

In Washington, U.S. Defense Secretary Chuck Hagel told Congress on November 13 that the U.S.-led coalition is making progress in the fight against IS in Iraq and Syria.

But Hagel said the United States must prepare for a long and difficult struggle.